Development Of Synthetic Unit Hydrographs For Watersheds In The Upper Awash And Upper Tekeze Basins

Hydro-meteorological data are very indispensable for the assessment and development of waterrnresources. However, most catchments in Ethiopia are ungauged and direct streamflowrnobservations are not available at most sites for which rainfall–runoff relationships are required.rnTherefore, if there are no flow data from a catchment, a technique is needed for estimating thernrequired design parameters, which do not require the availability of hydrological records. Onernoption is to develop models for gauged catchments and link their parameters to physicalrncharacteristics, so that the approach can be applied to ungauged basins in the region, whosernphysical characteristics can be determined. Usually, one resorts to Synthetic Unit Hydrographsrn(SUHs) if there are no observed discharge hydrographs. This requires that the catchmentrncharacteristics be obtained or determined which are then used to adapt the SUH to suit arnparticular catchment.rnIn this research work coefficients required for the construction of synthetic unit hydrographsrnusing Snyder’s method for watersheds, which have areas in the range from 59.8km2 to 2449km2rnin the upper Awash and Tekeze basins, have been determined. Moreover equations forrnestimating the lag time coefficient (Ct) and peak discharge (qp) from watershed characteristicsrnwere also developed.rnThirty concurrent rainfall-runoff events from six gauaged watersheds, four in the upper Tekezernand two in upper Awash, were used to estimate the coefficients and develop the equations. 27rnrainfall-runoff events were used for calibration and three were used for verification. Due to thernlack of getting watersheds with adequate rainfall and runoff records in Tekeze basin twornwatersheds that have almost similar hydrological characteristics were used to produce arnreasonable and largely reliable estimate.rnLag time coefficient (Ct), peak discharge coefficient (Cp), unit hydrograph widths coefficientsrn(Cw) at 50% and 75% of the peak and base time coefficient (Cb) were determined by calibratingrnSnyder’s equations with the available rainfall- runoff data. The mathematical relations for lagrntime coefficient (Ct) and peak discharge (qp) with watershed characteristics were established byrnusing regression analyses.rnxiiirnLag time coefficient (Ct) for the gauged watersheds range from 0.362 to 0.736 with mean valuernof 0.542 and a standard deviation of 0.157. The peak coefficients of the unit hydrographs of therngauged watershed range from 0.064 to 0.346 with mean value of 0.180 and a standard deviationrnof 0.112. Coefficients for the base time (CB) and for the widths of unit hydrograph (Cw) at 50%rnand 75% of the peak discharge of the gauged catchments are found to be 1.006, 0.20 and 0.108rnrespectively these values are recommended to construct UH for an ungauged watershed inrnupper Awash and upper Tekeze basins.rnLag time is an essential input for Snyder’s synthetic unit hydrograph model. Strong correlationrnwith R2 = 0.90 is seen between Lag time coefficient (Ct) and slope of a watershed. And hencernthe equation Ct = 0.032*S-0.597 is recommended to estimate lag time coefficient (Ct) for anrnungauged watershed in the upper Awash and Upper Tekeze basins. In addition to this lag timerncan be estimated directly from the physical characteristics of the watershed by using thernequation tp = 0.127*(LLc/S0.5) 0.352. Strong correlation with R2= 0.92 is seen also between qp (inrnm3/s) and the variable Z = A/tl and hence the formula qp = 8.71*10-3 *(Z) 1.78rnCan be used to estimate the peak discharge.rnTherefore, applying the equations and the coefficients give an estimate to the required UHrncharacteristics, which might serve the intended purpose, as long as the hydrometeorological andrnphysical characteristics of the watershed under consideration are with in the range of thoserncharacteristics for the watersheds in this study.

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